Chthon by Piers Anthony (Ballantine U6107 – 1967)

Aton’s crime was simple. he loved a minionette. All worlds despised and feared this almost mythical siren but no creature anywhere could tell him why.
In the brutally hot, subterranean garnet caves of Chthon to which he had been condemned, Aton sought an answer.
And when he could not find it – he determined to seek it elsewhere.
No man had ever escaped Chthon and lived. But the mionionette called. Aton had to have his answer.
So did Chthon…